70 MYID.E. 



shell is wedge-like on the longer side, with the terminal 

 edges reflected outwards : until it arrives at maturity 

 the truncation is incomplete. This alteration of shape 

 is not caused by absorption, but by the formation of 

 additional layers in front, which make the shell propor- 

 tionally longer or deeper than it previously was. The 

 Arctic variety, to which Forbes gave the name of Udde- 

 vallensis, is the usual form in glacial deposits ; it is 

 more depressed in the middle, obliquely truncated in- 

 wards, and excavated at the posterior end, frequently 

 to so great an extent, and in such a fashion, as if there 

 were cut 



"A huge half-moon, a monstrous cantle out." 



The internal structure of the shell is distinctly seen in 

 fossil specimens of this variety which have been perfo- 

 rated by the Cliona. A section thus exposed shows at 

 least 18 layers, and is unequally eroded, so as to re- 

 semble in miniature a perpendicular rock of oolite with 

 caverns at its base. A specimen of an intermediate 

 form, which I lately dredged in Dourie voe, Shetland, 

 measures 3| inches in breadth, and is of proportionate 

 length. 



Petiver called this shell " The broad Pholade-muscle"; 

 when half-grown it is the M. ovalis of Turton, and M. 

 pullus of S. Wood; the young is the Sphenia Sivainsoni 

 of Turton, and M. Swainsonii of Loven. 



3. M. Binghami"*, Turton. 



Schema Binghami. Turt. Dith. p. 36, t. 3. f. 4, 5, and 1. 19. f. 3. Sphamia 

 Binghami, F. & II. i. p. 190, pi. is. f. 1-3, and (animal) pi. T. f. 3. 



Body elongated and compressed, pale yellowish-white : tubes 

 short, especially the incurrent one ; mouth of each encircled 

 by 5-10 rough white cirri ; valve of excretal tube large and 



* Named after the late Lieut. -General Bingham, an assiduous collector 

 of British shells. 



